1. Field of the Invention
This invention relates generally to an electric positional actuator and, more particularly, to an electric positional actuator employing a default positioning device for returning an actuated device to a desired default position in the event of actuator failure, where the actuator has particular application for controlling air flow through a turbocharger or a supercharger.
2. Discussion of the Related Art
In a four-stroke internal combustion engine, the combustion air and fuel mixture typically enters the cylinders of the engine under atmospheric pressure. By pressurizing the combustion air before it enters a cylinder, more fuel can be mixed with the high-pressure air to obtain the desired air/fuel mixture, and thus, more power can be delivered for each stroke of the cylinder. A supercharger employs a compressor driven by the engine to increase the combustion air pressure. However, the power increase from the cylinders is partly lost due to the parasitic losses from driving the compressor by the engine. A turbocharger uses the exhaust gas pressure to drive a turbine. A compressor mounted on the same shaft as the turbine is rotated by the turbine, and is thereby used to increase the combustion air pressure. Thus, the compressor is not coupled to the engine, and the losses associated therewith are avoided.
Control valves are employed in a supercharger and a turbocharger to control the flow of combustion air through the compressor. One design employs a series of vanes that control the back-pressure in the turbine of a turbocharger to control turbine speed. Other turbocharger or supercharger designs employ a valve flapper member that controls air flow through the turbine or compressor. A suitable actuator is used to position the valve member or the vanes in the desired location. It would be desirable to provide a default device within the actuator so that the valve member or vanes remain at a desirable position in the event of actuator failure so that the engine keeps running.
U.S. Pat. No. 5,492,097 issued Feb. 20, 1996 to Byram et al. discloses a throttle body valve for regulating the flow of combustion air to an internal combustion engine. The valve includes a valve member selectively positionable between a minimum air flow position and a maximum air flow position in a combustion air passage extending through the valve. A default position is defined between the minimum and maximum air flow positions to allow the engine to operate if the actuator fails. A first end of a biasing member applies a force against the valve member towards the default position when the valve member is in the minimum air flow position, and a second end of the biasing member applies a force against the valve member towards the default position when the valve member is in the maximum air flow position.
In accordance with the teachings of the present invention, an electric positional actuator is disclosed that includes a default actuation device for positioning the actuated device in a default position in the event of actuator failure. The actuator has particular application for controlling air flow in a turbocharger or supercharger, but can be used for controlling many other devices and systems. The actuator includes an electric motor that controls the rotational position of a shaft through a gear system. When the shaft rotates, it moves a link-bar that actuates the actuated device. The actuator further includes a printed circuit board having a microprocessor and related circuitry. External control signals cause the microprocessor to activate the motor to position the shaft at the desired location. A rotational sensor coupled to the circuit board detects the position of the shaft, and provides a feedback signal to the microprocessor of the shaft""s position.
The default device positions the shaft in a default position in the event of actuator failure. The default device includes a spring wrapped around the shaft. One end of the spring is positioned on one side of a lever arm coupled to the link-bar, and an opposite end of the spring is positioned on the other side of the lever arm. Therefore, the shaft rotates against the bias of the spring in both directions. If motor power is not applied to the shaft, then the spring holds the shaft in the default position.